Hydrodynamic torque converters in automatic transmissions chiefly perform the task of making possible a comfortable starting operation with an increased start-up torque and to uncouple the torsional vibrations of the engine from the drive line. With the constantly increasing demands for reducing both fuel consumption and gas emission, it has become necessary further to reduce the losses in the hydrodynamic torque converter. In the first place, this is achieved by optimizing the torque converter itself and by using a (slip regulated) lock-up clutch. The latter objective is to reduce the share of the comfortable but dissipative hydrodynamic power transmission and thus to spare fuel without giving up a sufficient uncoupling of vibrations.
The control of the lock-up clutch essentially consists of a strategy and pressure modulation. In the strategy established is which state the torque converter lock-up clutch assumes, by taking into consideration an optimal consumption and comfort characteristic. At least two basic states (open and closed) have been defined. Together with that, a slipping operation of the torque converter lock-up clutch can be implemented.
DE-A 41 04 542 has disclosed a control system for control of a lock-Lip clutch in a continuously variable transmission. For control a ratio of the speeds of rotation of the input engine and of the primary pulley of the continuously variable transmission is formed. If this value is one, the torque converter lock-Lip clutch is closed. By comparing an actual ratio between the primary and secondary pulleys of the continuously variable transmission with the computed theoretical ratio, it is determined that the torque converter must be activated. In this case, a disengagement signal is generated to disengage the torque converter lock-up clutch. With the engagement and disengagement, time steps become effective in order to ensure that the secondary pressure is adapted to the torques to be transmitted--in accordance with the operating state of the torque converter lock-up clutch. The strategy takes into consideration the basic, functions for control of a torque converter lock-up clutch.